The present invention relates generally of? an image projection apparatus, such as a liquid crystal projector (“LCP”), which uses a reflection type liquid crystal display (“LCD”) device, and more particularly to an adjustment method of a 1/4 phase or wave plate.
Due to the recent spread image projection apparatuses, such as LCPs, high-quality image projection apparatus have been increasingly demanded. The recent polarization type LCP modulates illumination light from a lamp at a reflection type LCD device or reflection type liquid crystal panel, analyzes the modulated light at a polarization beam splitter (“PBS”), and introduces the analyzed light to a projection optical system. In order to improve the contrast of a colored image, a 1/4 phase or wave plate is arranged between the PBS and the reflection type LCD device. See, for example, Japanese Patent Publication, Application No. 2004-12864.
This prior art reference splits the optical path of the white light from the lamp into two optical paths. The first optical path introduces the first colored light to a first PBS, and the second optical path introduces second and third colored lights to a second PGS. A third PBS synthesizes outputs from these two PBSs. Red, green and blue (“RGB”) may be arbitrarily selected from among the first to third colored lights.
Other prior art include Japanese Patent Publications, Application Nos. 2001-228534, 2002-062583, 2002-207213, and 2003-035886, and Japanese Patent Publication No. 07-038050.
The conventional LCP suffers image degradations due to the leakage light. A description will now be given of the “leakage light.” For example, assume a reflection type LCD device that rotates a polarization direction of the light to be projected onto a screen by approximately 90° (but maintains the polarization direction of the light in displaying black) and maintains the polarization direction of the light that is not to be projected onto the screen. In this reflection type LCD device, the “leakage light” is the light that is projected onto the screen from among the lights incident upon the area that is controllably prohibited to introduce the light to the screen side. In other words, the “leakage light” is light that is projected onto the screen even when the polarization direction is so controlled in the reflection type LCD device that the reflected light does not enter the screen.
The above prior art reference proposes to adjust the 1/4 wave plates such that the contrast is maximum or the leakage light intensity is minimum.
However, it is difficult to eliminate the leakage light completely. In particular, use of a color-selecting phase plate would increase the leakage light intensity on an optical path of a color or the light in a specific wave range, to which the color-selecting phase plate is applied. Then, the leakage light ratios among the RGB may possibly differ: Black is tinted with a color of the light having the largest leakage light intensity and that color is different from white. Here, the color-selecting phase plate is serves to rotate, by 90°, a polarization direction of the light in a predetermined wave range in the visible light wave range, and maintain the polarization direction in other wave ranges.
Tinted black, such as magenta black, can be electrically converted into nearly black. FIG. 7 schematically shows a screen of the LCD device in this method. A screen 10 has an effective part or area 12 that displays an image by electrically controlling the liquid crystal and modulating the incident light, and a peripheral part or area 14 that encloses the effective part 12 and does not electrically modulate the liquid crystal. A transmission type LCD device would not project an image on the peripheral part 14 but project an image on the effective part 12 onto the final screen, since the peripheral part 14 is outside the effective area 12 in the LCD device and blocks all the lights in principle. However, the reflection type LCD device would project the peripheral part 14 onto the final screen, because it reflect part of the light incident upon the peripheral part 14 to the projection optical system, although the peripheral part 14 is outside the effective part 12. The electrically black convertible area is an area corresponding only to the effective part 12. Thus, after the electric black conversion, black is displayed or projected on the area corresponding to the effective part 12. However, the peripheral part 14 remains, for example, in reddish or magenta black (due to a comparatively large amount of the reddish leakage light), although it may, of course, be greenish or bluish black. However, a difference in black between the effective part 12 and the peripheral part 14 stands out. In particular, when the peripheral part 14 is in magenta black and the effective part 12 displays black or dark images, magenta black in the peripheral part 14 is undesirably stressed.